1. Field of the Invention
The present invention relates to a high-frequency superimposing method for superimposing a high-frequency component on a semiconductor laser drive current when recording information on an optical disk in an optical disk drive and to an optical disk drive using the high-frequency superimposing method. More specifically, the present invention relates to a high-frequency superimposing method capable of high-speed, high-density recording and an optical disk drive using the same.
2. Description of the Related Art
Among recordable optical disks there are write-once type optical disks that allow the user to record data only once (e.g., CD-R and DVD-R) and rewritable type optical disks that permit rewriting a number of times (e.g., CD-RW and DVD-RW).
To start recording data, the optical disk drive needs to position its optical head at a desired track (groove) on a disk and start recording from a desired position (tangential direction position). This positioning of the head is done by reading address information marked in advance on the disk. To make the drive compatible with read-only disks (CD-ROMs and DVD-ROMs), address information corresponding to the recording position on the disk is added to user data when the user data is recorded on the disk.
In the case of CD-R and CD-RW, the address information on the disk is formed in units of frame (equivalent to 1/75 second for a normal speed playback) by slightly wobbling the groove in a radial direction (a reproduced signal from the wobbled groove is hereinafter referred to as a wobble signal) and modulating a wobble frequency. In the case of DVD-R and DVD-RW, although the groove is slightly wobbled in the radial direction, the wobble frequency is not modulated. Instead, tiny dents called land pre-pits (LPPs) are located at predetermined positions along the groove wobble so that the presence or absence of these pits forms address information.
To recognize positions on the disk correctly for information recording, the groove wobble must be read (i.e., address information must be identified) with high precision also during recording. However, since a semiconductor laser is affected by a reflected light from the disk, a resonance state of the interior of the semiconductor changes, resulting in a change in its laser power. That is, a so-called optical feedback causes noise. Semiconductor lasers capable of generating a high output, in particular, are more likely to be affected by the optical feedback than read-only semiconductor lasers with a low output (High-power semiconductor lasers have a reduced reflectivity at the laser end facet to allow an increased power to be output to the outside. Hence, the reflected light easily enters through this end facet into an oscillation portion in the interior of the laser, changing the resonance state and making it more likely for noise to occur.)
To reduce noise of such a semiconductor laser there has been a high-frequency superimposing method that involves superimposing a high-frequency component on a drive current of the semiconductor laser. For example, U.S. Pat. No. 6,421,314 discloses a method in which not only is a high-frequency signal superimposed on a semiconductor laser drive current during playback but the high-frequency signal of an optimum frequency and/or amplitude for forming spaces (erasing) and marks is superimposed during recording by using a common high-frequency superimposing circuit.